les, light: LES/2 protocol version (#14970)

This PR implements the new LES protocol version extensions:

* new and more efficient Merkle proofs reply format (when replying to
  a multiple Merkle proofs request, we just send a single set of trie
  nodes containing all necessary nodes)
* BBT (BloomBitsTrie) works similarly to the existing CHT and contains
  the bloombits search data to speed up log searches
* GetTxStatusMsg returns the inclusion position or the
  pending/queued/unknown state of a transaction referenced by hash
* an optional signature of new block data (number/hash/td) can be
  included in AnnounceMsg to provide an option for "very light
  clients" (mobile/embedded devices) to skip expensive Ethash check
  and accept multiple signatures of somewhat trusted servers (still a
  lot better than trusting a single server completely and retrieving
  everything through RPC). The new client mode is not implemented in
  this PR, just the protocol extension.

ethstats/ethstats.go

@@ -379,7 +379,7 @@ func (s *Service) login(conn *websocket.Conn) error {
 		protocol = fmt.Sprintf("eth/%d", eth.ProtocolVersions[0])
 	} else {
 		network = fmt.Sprintf("%d", infos.Protocols["les"].(*eth.EthNodeInfo).Network)
-		protocol = fmt.Sprintf("les/%d", les.ProtocolVersions[0])
+		protocol = fmt.Sprintf("les/%d", les.ClientProtocolVersions[0])
 	}
 	auth := &authMsg{
 		Id: s.node,